MEPX
Chapter 3 of 10All chapters
  1. 1Describing motion
  2. 2The laws of motion
  3. 3Free body diagrams
  4. 4Friction and normal forces
  5. 5Equilibrium and torque
  6. 6Work and energy
  7. 7Momentum and collisions
  8. 8Circular motion
  9. 9Projectiles
  10. 10Solving problems

Chapter 3 of 10

Free body diagrams

The habit that prevents mistakes.

How to draw one

Reduce the object to a dot. Draw every force acting on it, labelled, with direction. Include weight, normal force, friction, tension and applied forces, and nothing else.

  • Never draw a force the object exerts on something else.
  • Centrifugal force does not belong: what acts is the centripetal force inwards.

Then resolve

Choose axes that suit the problem, often along a slope rather than horizontal, and split each force into components. The algebra afterwards is usually trivial.